1. Field of the Invention
The present invention relates to a water disintegratable sheet of which fibers can be dispersed in a large amount of water, more particularly, relates to a water disintegratable sheet which can offer a good balance of strength and water disintegratability.
2. Description of the Related Art
It is preferred that wet sheets for wiping a discharging part of a human body and wet wipers for cleaning a toilet are disintegratable in water. In absorbent articles such as sanitary napkin, panty liner and disposable diaper, it is also preferred that a topsheet covering a top surface of an absorbent layer and a backsheet covering a bottom surface of the absorbent layer are disintegratable in water. In addition, packaging sheets for packaging such absorbent articles are also preferably disintegratable in water.
If these sheets are disintegratable in water, they can be disposed of in a flush toilet after use. Such water disintegratable sheet disposed of in a flush toilet is immersed in a large amount of water in a flush toilet or a septic tank, and constituent fibers of the water disintegratable sheet are dispersed in water, thereby preventing the sheet from floating and remaining in a septic tank.
In such water disintegratable sheet, dry strength should be excellent as well as wet strength should be increased to some extent. When put in a large amount of water, on the other hand, the constituent fibers should be rapidly dispersed.
Japanese Unexamined Patent Publication No. H9-228214 (1997/228214) discloses a water disintegratable nonwoven fabric, which is produced by blending pulp with regenerated cellulose fibers having a fiber length of 4 to 20 mm, forming a web on a wire belt of a paper machine, and entangling the fibers about each other through a water-jet treatment. In this water disintegratable nonwoven fabric, the wet strength is increased by entangling the long regenerated cellulose fibers about each other through the water-jet treatment, as well as the dry strength is increased by entangling pulp about each other and about the regenerated cellulose fibers. On the other hand, when the sheet formed by entangling the fibers is immersed in a large amount of water, the fiber entanglement is loosened so that the sheet can be disintegrated in water. It is also disclosed that fibrillated pulp may also be used for increasing the strength of the water disintegratable nonwoven fabric.
The water disintegratable nonwoven fabric disclosed in the above-mentioned Publication is intended to obtain the sheet strength and the water disintegratability by entanglement of the regenerated cellulose fibers and pulp. In such fiber entangled nonwoven fabric, however, it is difficult to improve both the sheet strength and the water disintegratability. For example, if long regenerated cellulose fibers having a fiber length of about 20 mm are used, the fibers are excessively entangled through the water-jet treatment, deteriorating the water disintegratability. Conversely, if short regenerated cellulose fibers having a fiber length of about 4 mm are used, the fibers cannot be sufficiently entangled, deteriorating the sheet strength.
The above-mentioned Publication also discloses the use of fibrillated pulp for increasing the strength of the nonwoven fabric. However, pulp is as short as 4 mm or less, and it will be cut into small short pieces when fibrillated by beating. The fibrillated pulp thus cut into small short pieces cannot sufficiently bind the regenerated cellulose fibers and pulp, because it is too small and short.
In addition, since beaten pulp is too small and short, it easily drops out of a fibrous web during the web formation or the water-jet treatment, thereby deteriorating the yield. On the other hand, if the fibrillated pulp thus cut into small short pieces is agglomerated in the fibrous web, the dispersibility of the regenerated cellulose fibers is deteriorated, thereby making it difficult to form a nonwoven fabric in which fibers are uniformly distributed.